Back contact solar cells on monocrystalline n-Si wafers have been developed by different solar cell manufacturers for a number of years, and some of these cells are already available on the market.
For example, reference is made to the so-called A300 cell by SunPower (cf. W. D. Mulligan, D. H. Rose, M. J. Cudzinovic, D. M. DeCeuster, K. R. McIntosh, D. D. Smith, R. M. Swanson, “Manufacture of solar cells with 21% efficiency”, Proceedings of the 19th European Photovoltaic Solar Energy Conference, Paris, France (2004)). The A300 cell is a so-called interdigitated back contact cell (IBC), which means that both the emitter and the BSF (back surface field) or base contact strips are situated on the rear side of the cell and are developed in the form of two meshing fork structures.
The required electrical separation of adjacently located n-doped and p-doped semiconductor regions on the same surface may be achieved in different ways. For example, there is the possibility of placing the two regions at different levels by removing the silicon oxide precipitated on the rear surface around the regions provided as base contacts using laser ablation (P. Engelhardt, N.-P. Harder, T. Neubert, H. Plagwitz, B. Fischer, R. Meyer and R. Brendel, “Laser Processing of 22% Efficient Back-Contacted Silicon Solar Cells”, 21st European Photovoltaic Solar Energy Conference, Dresden, 2006, p. 1).
Once the surface damage caused by the laser process, and approx. 20 μm of the silicon have been removed by wet-chemical etching, the emitter doping with phosphorus into the deeper-lying regions of the rear side, the front side and the connecting holes between front emitter and rear side emitter is implemented simultaneously, with the aid of a standard POCl3 process.
The metallic coating of both regions then takes place in a single aluminum vapor deposit step, the contact regions being electrically separated from each other by tearing the thin metal layer at the produced, virtually perpendicular step structure in the semiconductor surface.
Technologies for the production of passivated emitters and of local spot contacts to the two semiconductor regions of base and emitter are likewise known and acknowledged in the literature (cf. R. A. Sinton, Y. Kwark, R. M. Swanson, “Recombination Mechanisms in Silicon Solar Cells”, 14th Project Integration Meeting, Photovoltaic Concentrator Technology Project, June 1986, p. 117-125).
The local opening of the passivation layer, which simultaneously is the insulation between the semiconductor regions and the superposed metallic current paths, is increasingly implemented with the aid of lasers. On the one hand, so-called laser ablation is employed for removing the insulation layer only locally. On the other hand, the so-called laser-fired contact method (LFC) is employed, in which laser flashes move the vapor-deposited or sputtered aluminum layer through the insulating layer in order to contact the semiconductor regions lying underneath.
From DE 696 31 815 T2, it is known to use an AlSi eutecticum as conductor base for the p-emitter structure, which is produced on the surface once the aluminum has diffused into the silicon through a previously inwardly diffused n+ layer of the rear side. The solution there also uses screen printing of aluminum paste through oxide windows above the n-base regions. The disadvantage of this solution is that the aluminum doping and the contacting of the aluminum emitter must be implemented in one step, i.e., across a large surface, so that the surface of the emitter and the surface of the metal contacting are identical. Thus, no passivation of the emitter with local contacts is possible. This results in a large surface recombination rate and thus relatively low efficiency.
Both the laser ablation and the LFC method for the production of local contacts have the disadvantage that these methods are of sequential nature. In other words, the holes for each wafer must be produced individually, one after the other.